Pump

ABSTRACT

A pump for carrying supercritical CO 2  fluid or liquid CO 2  capable of securing reliability by optimizing a bearing part. An angular ball bearing ( 8 ) comprises an inner ring ( 8   a ), an outer ring ( 8   b ), balls ( 8   c ) held therebetween, and a cage ( 8   d ) holding the balls ( 8   c ). Both the balls and the inner and outer rings are formed of a ceramic to form a totally ceramic bearing. Also, the cage ( 8   d ) is guided by the outer ring or the inner ring. Specifically, for example, when the cage is guided by the inner ring, the inner peripheral surface ( 8   db ) of the cage ( 8   d ) opposed to the outermost peripheral surface ( 8   ab ) of the inner ring ( 8   a ) is guided by the outermost peripheral surface ( 8   ab ) of the inner ring ( 8   a ).

TECHNICAL FIELD

The present invention relates to a pump which carries supercritical CO₂fluid or liquid CO₂.

BACKGROUND ART

Because supercritical fluid can continuously change the volume (density)thereof by regulating the temperature and pressure, values of variousproperties such as dissolution characteristic and the like can be easilycontrolled, which makes it possible to utilize supercritical fluid as arefrigerant. Additionally, because the viscosity thereof is low but thediffusivity thereof is high, supercritical fluid is expected to be usedfor washing; or is expected to be used as a refrigerant because thethermal conductivity thereof is high.

Being easy to get and good in handling and the like because of nothaving explosive nature, inflammability and the like, water, CO₂ and thelike are generally used as supercritical fluid. Especially, CO₂ hascharacteristics that the critical temperature is 31° C. and the criticalpressure is 7.38 Mpa; that the temperature and the pressure are low butmanageability is superior, being compared with other materials; that CO₂is a gaseous body at the atmospheric pressure and the ambienttemperature; and the like. Therefore, CO₂ is made wide use of as anextracting solvent for materials which are easily affected by heat (suchas foods, flavoring ingredients and the like) and as a cleaning agentand the like which utilize the drying characteristics and permeabilitythereof. Additionally, CO₂ is generally used by being pressurized up tono less than 7 to 20 MPa in an extracting equipment or a cleaningequipment that employs the supercritical CO₂.

Because a pump for carrying CO₂ in such an equipment as mentionedhereinabove is used at high pressure, so-called sealless canned motortype of pump is adopted. In some cases, ball bearings are employed asthe bearings and used in solution (supercritical CO₂). However, when thepump is used for the food sector or semi-conductor sector, wear of thebearings cannot be permitted in order to prevent foreign objects(particles) from occurring inside the pump.

As an example of the pump, a turbo-molecular pump is disclosed in PatentLiterature 1. In addition, a canned motor type of pump is disclosed inPatent Literature 2. As for the rest, a semi-conductor manufacturingmethod and a semi-conductor manufacturing equipment are disclosed inPatent Literature 3, which prevent heavy-metal contamination ofextra-pure water being manufactured by an extra-pure water manufacturingsystem for manufacturing of semi-conductors.

Patent Literature 1: Patent Application Laid Open as H10-274188

Patent Literature 2: Patent Application Laid Open as 2000-9077

Patent Literature 3: Patent Application Laid Open as 2003-124177

DISCLOSURE OF THE INVENTION

Issues to be Solved by the Present Invention

Because ball bearings are used in supercritical CO₂ (or liquid CO₂) oflow viscosity, lubrication by liquid being pumped cannot be expected andthe environmental condition is severe for the wear of the bearings.Additionally, in a cleaning equipment and the like, in order to enhancethe cleaning capability thereof further, there is a case where cleaningchemical is mixed into CO₂. Therefore, there is a concern of corrosionand the like of the ball bearings being caused by chemicals.

The above-mentioned ball bearings receive the radial load and thrustload which act on a rotor. Additionally, a pump is constructed in amanner that the preload is supplied by a bearing preload spring which isinstalled to the bearing on the shaft-end side, being opposite to thebearing on the impeller side which will be described later, so as toprevent so-called revolution skidding (side skidding) of the bearings.However, because the pump being so constructed as described above isused in the liquid being pumped (what is called “in a condition of beingthoroughly immersed”), rotational resistance (a loss due to agitation)of balls and a cage is large so that the revolution skidding of bearingsis easy to occur. Therefore, there arises a case where it is necessaryto apply a large preload in order to prevent the revolution skidding.

On the other hand, in order to secure the life of the bearings, it isdesirable to make the bearing load including the preload as small aspossible. However, because the bearings are used in the liquid beingpumped, the pump must be designed, taking prevention of the revolutionskidding into consideration.

It is an object of the prevent invention to provide a pump for carryingsupercritical CO₂ fluid or liquid CO₂ capable of securing reliability byoptimizing a bearing part.

Means to Solve the Issue

In order to achieve the above-mentioned object, a pump for carryingsupercritical CO₂ fluid or CO₂ liquid in accordance with the presentinvention has bearings which axially support the main shaft of a motordriving the pump, wherein inner rings, outer rings and balls (rollingelements) thereof are formed of a ceramic material, respectively.

Additionally, a cage holding the balls is guided by the outer ring,being guided by the inner peripheral surface of the outer ring, orguided by the inner ring, being guided by the outer peripheral surfaceof the inner ring; wherein the surface being guided by the outer ring orthe inner ring is both of the side surfaces or one side surface in theaxial direction.

Moreover, the ratio of the slot radius of the inner ring of the bearingis more than 52%, and the main shaft is hollow.

The curvature radius of the orbit slot of the inner ring heretoforeknown is no less than 52% as specified, for example, by the JISstandards. Especially, in a high-speed rotating range, antifrictionbearings that are used in the atmosphere of supercritical CO₂ fluid areeasy to wear, wherein the wear is presumed to be significantlyattributed to skidding caused by spinning. Balls of an angular ballbearing generally roll on the plane of the orbit of either the innerring or the outer ring almost without skidding, while spinning occurs onthe other plane of the orbit.

In addition, in the angular ball bearing, the curvature radius of theorbit slot of the inner ring is smaller than that of the outer ring, andthe major axis of the osculating ellipse is long when a load is applied.As a result, in the inner ring, spinning motion of the balls isdisturbed (being guided by the inner ring), which causes completespinning motion to occur on the orbit of the outer ring. However, whenthe rotation speed becomes high (and the load becomes small), effects ofa centrifugal force become large, easily causing the cage to be guidedby the outer ring.

In the present invention, attention is focused on the skidding due tospinning which significantly contributes to the wear and on thecurvature radius of the orbit slot of the inner ring, and by loweringthe PV value that is obtained by the skidding due to spinning (productof the bearing surface pressure “P” multiplied by the skidding speed“V”), the wear can be reduced. More specifically, by making thecurvature radius of the orbit slot of the inner ring larger than thestandard value so as to make the area of contact of the balls with theorbit slot small, the PV value is decreased.

Because the balls, the inner ring and the outer ring of the antifrictionbearing for the supercritical CO₂ pump in accordance with the presentinvention are formed of a ceramic, wear resistance is enhanced; wherein,additionally, the curvature radius of the orbit slot of the inner ringis larger than 52% of the ball diameter, and especially, larger than54%, the area of contact on the inner ring side becomes small. In theresult, the PV value becomes small, which makes it possible to reducethe wear further. Consequently, reduction in endurance of theantifriction bearing is prevented, so that a supercritical CO₂ pumpemploying the antifriction bearings will not cause a problem that itcannot be used continuously because the vibration becomes high.

In addition, the upper limit of the curvature radius of the orbit slotof the inner ring is not specifically limited, but is no more than 60%of the ball diameter, considering the other performances. Also, thecurvature radius of the orbit slot of the outer ring is not specificallylimited, but is generally more than 50.5% and not greater than 60%.

Moreover, the cage holding the balls is guided by a rotating ring. Or,the cage holding the balls is guided by a fixed ring, wherein the guidesurface is formed only on one side to the center of the balls in theaxial direction.

In order to have the guide surface of a fixed ring formed only on oneside in the axial direction, in case of being guided by the fixed ring,the inside diameter of the outer ring on one side in the axial directionmay be made large so as not to have the inner peripheral surface of thelarge diameter part of the outer ring contact the outer peripheralsurface of the cage; or the outside diameter of the cage on one side inthe axial direction may be made small so as not to have the outerperipheral surface of the small diameter part of the cage contact theinner peripheral surface of the outer ring. In addition, in case ofbeing guided by a rotating ring, the outside diameter of the inner ringon one side in the axial direction may be made small so as not to havethe outer peripheral surface of the small diameter part of the innerring contact the inner peripheral surface of the cage; or the insidediameter of the cage on one side in the axial direction may be madelarge so as not to have the inner peripheral surface of the largediameter part of the cage contact the outer peripheral surface of theinner ring.

In the antifriction bearing which is used in a fluid atmosphere,revolution of rolling elements (balls) [rotation of the cage] is delayedmainly by resistance of the fluid, especially in a high-speed rotationrange, causing skidding (revolution skidding) to occur between the orbitring and the rolling elements which must be in contact while rolling.When the revolution skidding occurs, the orbit ring and the rollingelements cannot sometimes be used continuously because they get damagedin a short time. In the present invention, an attention is focused onthis revolution skidding, and by restraining the revolution skidding,antifriction bearings can be used continuously.

Specifically, by having the cage guided by a rotating ring, delay inrevolution of the cage is decreased by a traction force due to contactwith the rotating ring and a traction force of the fluid in the guidepart (by driving the cage in the direction of revolution), therebyrestraining the revolution skidding. And also, by having the cage guidedby a fixed ring so as to have the guide surface formed only on one sideto the center of the rolling elements in the axial direction, theresistance which is applied to the rolling cage by the fixed ring(friction due to contact and viscosity resistance of the fluid) isdecreased, thereby restraining the revolution skidding.

Effects of Invention

In accordance with the present invention, a pump for carryingsupercritical CO₂ fluid or liquid CO₂, capable of securing reliabilityby optimizing a bearing part can be supplied.

Specifically, by employing a totally ceramic bearing, it is possible toenhance the corrosion resistance against a cleaning agent. Additionally,it is possible to reduce a centrifugal force due to high-speed rotation,enhance wear resistance and prevent damages caused by particles comingfrom the outside.

Moreover, by reducing the drag loss of the cage, damages caused by therevolution skidding of a bearing can be prevented. Also, by enhancingthe characteristic of the revolution skidding, the preload can bedecreased, which reduces the thrust load, thereby achieving extension ofoperating life of the bearing. Additionally, by decreasing the dragloss, efficiency of the pump can be enhanced.

For the rest, by optimizing the ratio of the ball diameter versus theradius of the orbit slot (ratio of the slot radius), seize resistancecan be improved and at the same time, extension of operation life can beachieved, thereby achieving optimization. Also, by applying a PEEKmember to the material of the cage, corrosion resistance and strength ofthe cage can be enhanced.

Moreover, when a totally ceramic bearing is employed, there is a concernof a damage being caused by excessive stress of shrinkage of the innerring of the bearing due to thermal expansion differential between themain shaft and the inner ring of the bearing that occurs when thetemperature ascends. However, this can be avoided by having the mainshaft formed to be a hollow shaft.

Also, by having the cage of an antifriction bearing guided by a rotatingring, a force driving in the direction of revolution from the fluid inthe rotating ring and the guide part acts on the rolling elements,thereby restraining the revolution skidding. In consequence, enduranceof the antifriction bearing can be prevented from deteriorating, so thatsuch a problem will not occur as a pump for supercritical CO₂ employingthis antifriction bearing cannot be used continuously because thevibration becomes higher.

Or, by having the cage guided by a fixed ring and having the guidesurface formed only on one side to the center of the rolling elements inthe axial direction, it is possible to reduce a force which disturbsmovement of the rolling elements to the direction of revolution, therebyrestraining the revolution skidding. In consequence, endurance of theantifriction bearing can be prevented from deteriorating, so that such aproblem will not occur as a pump for supercritical CO₂ employing thisantifriction bearing cannot be used continuously because the vibrationbecomes higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the construction of a CO₂ pumpin accordance with an embodiment of the prevent invention.

FIG. 2 is a schematic cross-sectional view showing a bearing inaccordance with the embodiment.

FIG. 3 is a longitudinal cross-sectional view showing an antifrictionbearing in accordance with the embodiment.

FIG. 4 is a longitudinal cross-sectional view showing an antifrictionbearing in accordance with the embodiment.

FIG. 5 is a longitudinal cross-sectional view showing an evaluationequipment of the antifriction bearing in accordance with the embodiment.

FIG. 6 is a graph showing the results of evaluation of the antifrictionbearing.

FIG. 7 is a graph showing the results of evaluation of the antifrictionbearing.

FIG. 8 is a schematic cross-sectional view showing the condition of themain shaft which is hollow.

DESCRIPTION OF CODES

1: Circulation Pump

2: Discharge-Side/Suction-Side Casing

3: Purging-Side Casing

4: External Cylinder

5: Manifold

6: Canned Motor

7: Main Shaft

8 and 9: Angular Ball Bearing

10: Impeller

11: Preload Spring

BEST MODE FOR CARRYING OUT OF THE INVENTION

Referring now to the drawings, an embodiment of the present inventionwill be described hereinafter. FIG. 1 is a cross-sectional view showingthe construction of a CO₂ pump in accordance with an embodiment of thepresent invention. A pump 1 comprises a discharge/suction-side casing 2,a purging-side casing 3 and an outer cylinder 4 being held therebetween.Outside the suction/discharge-side casing 2 is installed a manifold 5which sucks and discharges the liquid being pumped.

Inside the outer cylinder 4 is installed a canned motor 6 which drivesthe pump 1, comprising a stator 6 a outside thereof and a rotor 6 bbeing housed inside the stator 6 a. The rotor 6 b is installed to a mainshaft 7; and the main shaft 7 is supported on both ends thereof by anangular ball bearing 8 being installed to the discharge/suction-sidecasing 2 and an angular ball bearing 9 being installed to thepurging-side casing 3 so as to rotate, specifically, being axiallysupported.

Between the discharge/suction-side casing 2 and the manifold 5 isinstalled an impeller 10, which is mounted to one end of the main shaft7 and can rotate, working simultaneously with the main shaft 7. Themanifold 5 has a suction port 5 a for the liquid being pumped opened onthe extension shaft line from one end of the main shaft 7; and has aspiral casing 5 b mounted around the impeller 10. Additionally, adischarge port 5 c is opened from one point of the periphery part of apathway 5 b toward the outer peripheral surface of the manifold 5 in aradial direction.

On the other hand, the purging-side casing 3 has a purging port 3 aopened on the extension shaft line form the other end of the main shaft7, which discharges a part of sucked liquid being pumped. As for therest, a preload spring 11 is held between the purging-side casing 3 andthe angular ball bearing 9. The preload spring 11 is a corrugated platespring in the shape of a ring being located in the vicinity of the otherend of the main shaft 7 and provides an axial preload to the angularball bearing 9, serving as a constant-pressure spring method. Inaddition, the angular ball bearing 8 is referred as an impeller-sidebearing, while the angular ball bearing 9 is referred as ashaft-end-side bearing.

In the pump 1 as described hereinabove, when the rotor 6 b of the cannedmotor 6 and the main shaft 7 rotate, causing the impeller 10 to rotatesimultaneously, the liquid being pumped is sucked through the suctionport 5 a as shown with an arrow “A,” introduced into the pathway 5 b bythe centrifugal force of the impeller 10 and is discharged through thedischarge port 5 c in the end as shown with an arrow “B.” Additionally,a part of the liquid being pumped and sucked through the suction port 5a passes through the inside of the angular ball bearings 8 and 9 and thecanned motor 6, cooling them, and is discharged through the purging port3 a as a purging flow as shown with an arrow “C.”

Now, in the present invention, in order to enhance wear resistance andcorrosion resistance and reduce the centrifugal load during high-speedrotation, ceramic materials (for example, silicon nitride Si₃N₄, aluminaAl_(2 O) ₃, silicon carbide SiO₂ and the like) are employed for theinner and outer rings and the balls of the angular ball bearing. Byhaving the bearing members totally formed of a ceramic, wear resistanceagainst particles being brought from the outside is enhanced, andfurther, in application in which metal particles (particles of wear) arenot permitted, favorable effects can be achieved on prevention againstmetal pollution.

Additionally, the cage is designed in a manner that the drag loss(rotational resistance) becomes smaller. By this, it is possible toprevent the revolution skidding and reduce the preload (the thrustbearing load), so that the service life of the bearings can be extended.In order to secure corrosion resistance and wear resistance against thecleaning agent and maintain the strength for high-speed rotation, a PEEKmaterial (polyether ether ketone) is used for the material of the cage.As for this, by using the PEEK material containing 30% glass fiber, forexample, the strength is increased further. In addition, the ratio ofthe ball diameter versus the radii of the orbit slots of the inner andouter rings is optimized in order to enhance wear resistance anddetachment resistance in high-speed and high-load conditions. Moreover,by containing solid lubricants such as PTFE, carbon and the like, wearresistance of the ball bearings in supercritical CO₂ fluid or liquid CO₂which has inferior lubricating property is enhanced.

FIG. 2 is a schematic cross-sectional view showing the bearing inaccordance with the embodiment of the present invention. FIG. 2 shows acase of the angular ball bearing 8, but the angular ball bearing 9 isformed only in a manner that FIG. 2 is flipped horizontal, andbasically, has the same construction. As shown in FIG. 2, the angularball bearing 8 in accordance with the embodiment comprises an inner ring8 a, an outer ring 8 b, balls 8 c being held therebetween and a cage 8 dholding the balls 8 c.

The inner periphery of the outer ring 8 b consists of an orbit slot 8 bawhere the ball 8 c rolls and inner peripheral surfaces 8 bb on bothsides in the axial direction thereof. Also, the outer periphery of theinner ring 8 a consists of an orbit slot 8 aa where the ball 8 c rolls,the outermost peripheral surface 8 ab on one side in the axial directionand the outer peripheral surface 8 ac on the other side which continuesform the orbit slot 8 aa. Additionally, the cage 8 d is a ring-shapedmember being installed between the inner ring 8 a and the outer ring 8b, and at the predetermined locations over the whole periphery thereofare provided pockets 8 da having the ball 8 c caught therein, beingequiangularly spaced. In addition, “a” in the figure is a contact angle.

The ball bearing of a conventional CO₂ pump is totally formed ofstainless steal, or has only the balls formed of a ceramic but has theinner and outer rings formed of stainless steel. However, as describedhereinabove, the bearing in accordance with the embodiment is a totallyceramic bearing, wherein the balls and the inner and outer rings areformed of a ceramic. In addition, the cage of the conventional bearingis guided by both sides of the outer ring, being guided by the innerperipheral surfaces on both sides in the axial direction of the outerring. However, the cage of the bearing in accordance with the embodimentis guided by the inner ring, and more precisely, is guided by one sideof the inner ring, being guided by the outer peripheral surface on oneside in the axial direction of the inner ring.

Specifically, in FIG. 2, the inner peripheral surface 8 db of the cage 8d facing opposed to the outermost peripheral surface 8 ab of the innerring 8 a is guided by the outermost peripheral surface 8 ab of the innerring 8 a. This will enable the cage 8 d to rotate easily in accordancewith rotation of the inner ring 8 a, thereby decreasing the rotationalresistance (the drag loss) of the cage 8 d, which can achieve preventionof the revolution skidding of the bearing. Moreover, such means ispossible to reduce the drag loss as employing a totally ceramic bearinghaving no cage.

Additionally, seize resistance and extension of operating life of a ballbearing are contradictory. Therefore, the ball bearing in accordancewith the embodiment is designed in order to achieve both seizeresistance and extension of operating life by adjusting the ratio of theball diameter versus the radii of the orbit slots of the inner and outerrings (the ratio of the slot radius). The range of the ratio of the slotradius is more than “the slot radius/the ball diameter×100=52%.”Furthermore, it is desirable that the range of the ratio of the slotradius is within the range from 52 to 56% or is larger than 54%,especially.

The bearing in accordance with the embodiment of the present inventionwill be descried in details. The bearing in the accordance with theembodiment is an angular ball bearing as shown in FIG. 3, comprising anouter ring 112, an inner ring 113, balls 114 installed therebetween,serving as a plurality of rolling elements, and a cage 115 holding theballs 114. The outer ring 112, the inner ring 113 and the balls 114 areall formed of silicone nitride and the cage 115 is formed of PEEK, asuper engineering plastic.

The cage 115 is formed in a manner that the outside diameter thereof isslightly smaller than the inside diameter of the outer ring 112 (forapproximately 0.15 mm) in order to rotate smoothly, being guided by theouter ring 112 serving as a fixed ring. Then, the outer ring 112comprises a small diameter part 112 a which is the left half thereof,having the slightly larger inside diameter than the outside diameter ofthe cage 115 (for approximately 0.15 mm); and a large diameter part 112b which is the right half thereof, having the sufficiently larger insidediameter than the outside diameter of the cage 115; wherein the orbitslot of the outer ring 112 is formed so as to cover both the smalldiameter part 112 a and the large diameter part 112 b.

By this, the guide surface guiding the cage 115 is formed only on oneside of the outer ring 112 in the axial direction (on the left side tothe center of the balls). In addition, the “sufficiently large insidediameter” mentioned above means that when the outside ring 112 and thecage 115 rotate relatively, the distance between the facing surfaces ofthe outside ring 112 and the cage 115 in the left half is large enoughto have almost no traction act between the facing surfaces.

The antifriction bearing 111 shown in FIG. 4 is different from theantifriction shown in FIG. 3 only in the cage. The cage 116 has theinside diameter thereof increased to be slightly larger than the outsidediameter of the inner ring 113 in order to rotate smoothly, being guidedby the inner ring 113 serving as a rotating ring.

FIG. 5 shows an evaluation equipment of the above-mentioned antifrictionbearing 111. The evaluation equipment 121 evaluates the revolutionskidding of the antifriction bearing 111, comprising a cylindricalhousing 122 and a rotating shaft 123 being housed so as to rotaterelatively thereto, wherein the antifriction bearings 111 and 111 onboth right and left sides which support the rotating shaft 123,rotating, are installed at two locations being axially separated betweenthe housing 122 and the rotating shaft 123.

The rotating shaft 123 has the left end part thereof connected to thedriving part (not illustrated) of an air turbine. On the outer ring sidebetween the two antifriction bearings 111 exists a cylindrical coilspring 124 which loads the axial load thereto, while a cylindricalspacer 125 exists on the inner ring side. Then, a non-contactdisplacement gauge 126 is installed so as to face the right end surfaceof the antifriction bearing 111 on the right side.

The predetermined parts in the circumferential direction of theright-side surface of the cage 115 of the antifriction bearing 111 onthe right side is coated with aluminum, thereby enabling thedisplacement gauge 115 to detect the frequencies of the aluminum coatingpart of the cage 115, so that the rotation speed of the cage 115,namely, the revolution speed of the balls 114, can be obtained from thefrequencies.

By employing the above-mentioned evaluation equipment 121, therevolution speed “n_(C)” of the balls 114 of the antifriction bearing111 is measured; and by obtaining the “ratio of the revolution skidding(%)=(n_(T)-n_(C))/n_(T)×100” from the measured revolution speed “n_(C)”and the revolution speed “n_(T)” obtained theoretically, the revolutionskidding of antifriction bearings of different specification can beevaluated; wherein, “n_(T)” can be obtained by “n_(T)=the rotation speedof the inner ring×(1−the ball diameter×cos α/the pitch diameter of theball)/2,” where “α” is a contact angle. The ratios of the revolutionskidding obtained in the above-mentioned manner are shown in FIG. 6 andFIG. 7.

In the graph in FIG. 6, the white triangles (and the broken line “a”)show the ratios of the revolution skidding of the antifriction bearing111 being guided by one side of the outer ring (FIG. 3) when the axialload is 6.5 kgf; the white squares (and the broken line “b”) show theratios of the revolution skidding of the same bearing 111 when the axialload is 23 kgf; the black triangles (and the broken line “c”) show theratios of the revolution skidding of the antifriction bearing beingguided by both sides of the outer ring when the axial load is 6.5 kgf (acomparative example); and the black squares (and the broken line “d”)show the ratios of the revolution skidding of the same bearing when theaxial load is 23 kgf. As can be seen from the graph, in the antifrictionbearing 111 shown in FIG. 3, the ratios of the revolution skiddingbecomes small, compared with the antifriction bearing having the guidesurface formed on both sides in the axial direction, and the revolutionskidding that deteriorates the wear performance of the antifrictionbearing 111 is improved.

In the graph in FIG. 7, the white triangles (and the broken line “a”)show the ratios of the revolution skidding of the antifriction bearing111 being guided by one side of the inner ring (FIG. 4) when the axialload is 6.5 kgf; the white squares (and the broken line “b”) show theratios of the revolution skidding of the same bearing 111 when the axialload is 23 kgf; the black triangles (and the broken line “c”) show theratios of the revolution skidding of the antifriction bearing beingguided by both sides of the outer ring when the axial load is 6.5 kgf (acomparative example); and the black squares (and the broken line “d”)show the ratios of the revolution skidding of the same bearing when theaxial load is 23 kgf. As can be seen from the graph, in the antifrictionbearing 111 shown in FIG. 4, the ratios of the revolution skiddingbecomes significantly small, compared with the antifriction bearingbeing guided by the outer ring and having the guide surface formed onboth sides in the axial direction, and the revolution skidding thatdeteriorates the wear performance of the antifriction bearing 111 isremarkably improved.

FIG. 6 and FIG. 7 show the results of tests employing water. In theatmosphere of supercritical CO₂, gravity and viscosity become small,compared with water, so that effects can surely be expected. Therefore,in accordance with the bearings 111 shown in FIG. 3 and FIG. 4,endurance thereof is prevented from deteriorating, thereby making itpossible to operate the supercritical CO₂ pump employing theantifriction bearings as described continuously for a long time althoughthe bearings are subject to be a severe condition of being in thesupercritical CO₂ atmosphere.

The results of analysis of the properties of the ball bearing inaccordance with the embodiment will be descried hereinafter. Herein,effects of making the curvature radius of the orbit slot of the innerring 113 larger than 54% of the ball diameter will be described bygiving the results of calculation with an angular bearing having theinside diameter of Φ10 as a concrete example.

To put it simply, when the curvature radius of the orbit slot of theinner ring is changed from 52% to 56% of the ball diameter, the PV valuewill decrease from 747 (MPa·m/s) to 553 (MPa·m/s) where the axial loadis 50N, and the PV value will decrease from 935 (MPa·m/s) to 708(MPa·m/s) where the axial load is 100N and will decrease to beapproximately ¾ on an average. The PV value can be lowered in this way,thereby reducing the wear.

The above-mentioned effect can be achieved by making the area of contacton the side of the inner ring 113 small so as to lower the PV value in acase where the skidding occurs on the side of the inner ring 113. Byanalyzing in more details from this point of view, in order to lower thePV value effectively, it is desirable to make the curvature radius ofthe orbit slot of the inner ring larger than 54% of the ball diameter.The upper limit of the curvature radius of the orbit slot of the innerring is determined to be approximately as much as 60% of the balldiameter, considering the relation with the other performances. Thecurvature radius of the orbit slot of the outer ring is not limitedspecifically, but may be more than 50.5% and no greater than 60%.

In now, the supercritical CO₂ is generally used in the range from 35° C.to 100° C. On the other hand, there is a difference in the linearexpansion coefficient between the main shaft member of the motor and theinner ring of the bearing supporting the shaft member. To put it simply,because austenite stainless steel is generally applied to the main shaftmember, considering corrosion resistance, the linear expansioncoefficient of the main shaft member is large, while the linearexpansion coefficient of the inner ring of the bearing is small becausethe inner ring of the bearing is formed of a ceramic as describedhereinabove.

Therefore, there is a concern that the inner ring of the bearing may bedamaged due to expansion of the main shaft when the temperature ascends.Consequently, as shown in FIG. 8, by providing a bored hole 7 a to thecenter of the axis of the main shaft 7 serving as a hole for a mountingbolt of the impeller and making the main shaft 7 hollow, expansion ofthe main shaft 7 is let to go inward, thereby mitigating the stresswhich is generated in the inner ring 8 a of the angular ball bearing 8.Here, FIG. 8 shows the construction of the impeller side, and theconstruction of the shaft end side is the same although there are nobolts.

The bearing in accordance with the present invention employs balls forrolling elements thereof, so as to be a ball bearing, but not limitedto. The bearing may be a roller bearing that employs a cylindricalroller or a roller in the form of a circular truncated cone.

It is to be understood that the present invention may be carried out inany other manner than specifically described above as an embodiment, andmany modifications and variations are possible within the scope of theinvention.

1. A pump carrying supercritical CO₂ fluid or liquid CO2, wherein, abearing supporting a main shaft has an inner ring, an outer ring andballs thereof formed of a ceramic member, respectively.
 2. A pump asdescribed in claim 1, wherein, a cage holding the balls is guided by anouter ring, being guided by an inner peripheral surface of the outerring; or is guided by an inner ring, being guided by an outer peripheralsurface of an inner ring; and wherein, a guide surface of an outer ringor an inner ring is a surface on one side in axial direction.
 3. A pumpas described in claim 2, wherein, the cage is formed of PEEK member. 4.A pump as described in claim 1, wherein, ratio of slot radius of innerring of the bearing is more than 52%.
 5. A pump as described in claim 2,wherein, ratio of slot radius of inner ring of the bearing is more than52%.
 6. A pump as described in claim 3, wherein, ratio of slot radius ofinner ring of the bearing is more than 52%.
 7. A pump as described inclaim 1, wherein, the main shaft is hollow.
 8. A pump as described inclaim 2, wherein, the main shaft is hollow.
 9. A pump as described inclaim 3, wherein, the said main shaft is hollow.
 10. A pump as describedin claim 4, wherein, the main shaft is hollow.
 11. A pump as describedin claim 5, wherein, the main shaft is hollow.
 12. A pump as describedin claim 6, wherein, the main shaft is hollow.
 13. A pump as describedin claim 1, wherein, a cage holding the balls is guided by a rotatingring.
 14. A pump as described in claim 1, wherein, a cage holding theballs is guided by a fixed ring; and wherein, a guide surface is formedonly on one side to center of the balls in axial direction.
 15. A pumpas described in claim 2, wherein, the cage contains solid lubricant. 16.A pump as described in claim 3, wherein the cage contains solidlubricant.